1. Field of the Invention
This invention relates to the field of air pollution control and more particularly to the removal of particulate matter and noxious or otherwise objectionable or value-containing gases from gas streams resulting from the operation of various types of processes, including production of heat or power, industrial, chemical, combustion, material handling and other processes.
With the passage by Congress of the Clean Air Act of 1970 and similar legislation by many states and localities both prior to and subsequent to the federal legislation, national attention has been drawn to the problem of improving the ecology or of preventing further deterioration of the quality of the ambient air. Particulate matter, which may comprise solids or liquids, and various gases constitute the principal forms of pollutant material. These pollutants may be found in the gaseous effluent of, or may be generated as a by-product pollutant in, many processes. While industrial operations are an obvious source of much of the pollutants to be found in the air, additional sources include transport vehicles, such as automobiles, trucks, railroads, ships and aircraft and non-industrial operations, such as apartment houses and private dwellings, which may use coal or oil for heating or waste disposal purposes.
The pollutants found in the air may vary widely in form, size and chemical nature. For example, the particulate matter may be liquids or solids which, in turn, may be chemically active or inert. The particles may vary in size from substantially smaller than 0.01 micron up to a fraction of an inch and may include metal or mineral values of economic significance. The gaseous pollutants may be relatively innocuous gases, such as carbon dioxide, or highly toxic gases including gases such as hydrogen sulfide, sulfur dioxide, carbon monoxide, or various of the nitrogen oxides. Some of the gaseous pollutants may be further reacted in the atmosphere to form acids or other substances which may have deleterious effects on the environment.
Up to the present time, the principal effort in the control of air pollution has been directed at source control of particulates whereby the weight of the particulate emitted from a particular source, e.g. the stack of an industrial plant, has been limited to some small fraction of the weight of the total particulate emitted from the process being controlled. Although it has been possible on many occasions to obtain the desired level of emission on a weight or weight concentration basis, environmentalists have become aware that the quality of the ambient air has not improved and, frequently, there has been a degradation in the ambient conditions. This result may be explained in part by the fact that many sources of pollution, e.g. vehicles and residences, are controlled inadequately, if at all, so that an unacceptable quantity of pollutants is being discharged into the atmosphere.
However, another explanation of the continued high level of pollutants in the atmosphere is now available. It is known that, if particulate is large enough, it will settle out quickly under gravitational forces and will not produce a major air pollution problem except in the immediate vicinity of the point of emission. But, as the particulate becomes smaller, the residence time in the atmosphere increases greatly so that some submicronic-sized particulate may remain in the atmosphere for years. Such submicronic particulate, though constituting only a very small portion of the total weight of the emission, may represent the vast majority of the number of particles emitted and also may represent the vast majority of the total toxic material emitted. Thus, the contribution of the submicronic particulate to the degradation of the ambient atmosphere is disproportionate to its relatively small weight. As recognition of this effect grows, it is expected that legislatures and other control agencies will place greater emphasis on the removal of fine particulate.
In the past few years, the developing shortage of fuels, such as natural gas, low-sulfur coal and fuel oil, has caused an increased concern in the control of noxious gases and particularly sulfur dioxide since the substitution of fuels containing larger amounts of sulfur exacerbate the pre-existing problems of controlling sulfur dioxide and other sulfur containing effluents. An example of the effect of small amounts of sulfur dioxide in the ambient air (in the range of 40-100 parts per million) appears in the operation of commercial blowers or air compressors. Recent experience has shown increased maintenance costs due to the attack on bearings and other wearing surfaces by sulfuric acid formed from the sulfur dioxide contained in the air drawn through the blower. Existing equipment is technically and economically ineffective to control adequately these levels of pollution in the ambient air.
Another effect of the recent fuel shortage has been an escalation in the cost of fuels and particularly in the cost of low-sulfur fuels. The result has been an abrupt increase in the cost of energy in the form of electrical power, steam and compressed air. As the amount of energy required for pollution control equipment varies with the type of equipment, the increased energy costs have modified the competitive advantages of the various types of equipment available on the market. It is becoming increasingly apparent, however, that there is a great need to develop equipment and processes which are capable of removing pollutants, including very fine particulates and sulfur-containing gases, from gas streams with the expenditure of a minimum amount of energy. In addition, it is apparent that it is important to remove such pollutants at a minimum total annual cost to the user. This means that consideration should be given to the capital cost of equipment (amortized over the expected life of the equipment), the annual operating costs, particularly labor and energy costs, and the annual cost of maintaining the equipment.
2. Description of the Prior Art
The art has developed a number of different types of equipment over the years designed to remove particulate from gas streams, which equipment may be categorized in several ways.
One category of such equipment is fabric filters. In the filter separator a screen having interstitial openings of any desired size is placed as a barrier to the flow of the particulate-containing gas stream. A common form of the filter separator is known as a bag-house which comprises a large number of fabric bags of felt or woven fabrics having a fine mesh to trap the particulate from the gas stream. While the bag-house separator is one of the most effective of the prior art devices for the removal of fine particulate, it has several inherent disadvantages which prevent its adoption for many processes. First, the bag-house is a relatively large installation and may employ several thousand fabric bags. As a result of its complexity, the bag-house is expensive to install and maintenance and operating costs are high due to the necessity for frequent cleaning and replacement of the bags. Secondly, the operating temperature is limited by the nature of the fabric material so that cooling of the gases to be treated is frequently necessary. Finally, while the bag-house is quite effective for particulate removal down to a size of about 1 micron, it is not well adapted to the removal of pollutants such as sulfur dioxide where some type of chemical reaction is necessary nor to the removal of particles below 1.0 micron in size which may be found in fumes and smog.
Another commonly used device is the mechanical separator, the so-called cyclone or centrifugal separator. In this apparatus the particulate-containing gas is generally introduced tangentially into a cylindrical or conical vessel and, as the direction of the gas stream is changed, the particulate is separated therefrom. While the cyclone is effective for large particulate which will readily separate from a gas stream due to gravitational or inertial forces, its efficiency decreases with smaller particulate and becomes largely ineffective with respect to particulate which is less than about 10 microns in size. Also, the energy requirements of the cyclone are proportional to the pressure drop through the cyclone and increase rapidly as the particulate decreases in size.
A third category of gas cleaning equipment includes the precipitator which employs electrostatic forces. In this device, a particulate-containing gas stream is charged to one polarity and is then passed between oppositely charged plates which, in turn, attract the particulate. The particulate may then be removed by mechanical means. The electrostatic precipitator becomes largely ineffective for particulate less than about 2 to 3 microns in size. In addition to relatively high capital costs, the precipitator is expensive to operate and its performance tends to deteriorate in time. Where the effluent gas contains combustible material there may also be safety hazards which inhibit the use of the precipitator. Other inadequacies of the precipitator include the inability to remove sulfur dioxide and sensitivity to particulate resistivity.
Some separation may result from the action of gravitational forces though, in the above equipment, these forces were not intentionally exploited. Thus, if desired, a particulate-containing gas stream may be introduced into a large settling or stilling chamber where the velocity is reduced essentially to zero. Again, this device is most effective for large particulate. As the particulate becomes smaller, the time required for settling increases.
Although most of the equipment referred to above is of the dry type, wet separation equipment is also available. A simple type of wet scrubber is the spray chamber or atomizer wherein the particulate-containing gas is sprayed with a liquid to wet and capture the particulate. See, for example, the early U.S. Pat. Nos. 467,264, 605,280 and 798,287. Basically, the wet scrubber removes particulate by a process of collision between liquid droplets and particles. In order to increase the probability of such collisions, the number of droplets available for collision and the relative velocities of the particle and droplet should be maximized. Recognition of this fact led to the development of the venturi scrubber wherein the particulate-containing gas was accelerated to a high velocity in a venturi tube and the water injected through spray nozzles located in or adjacent to the throat portion of the venturi. Recent examples of such venturi scrubbers are shown in U.S. Pat. Nos. 3,490,204, 3,567,194, 3,582,050 and 3,812,656. Normally, the particulate-containing gas is driven through the venturi by fans, blowers or ejectors which may be located either upstream or downstream from the venturi scrubber. Many forms of the venturi scrubber exist, characterized by differences in the way in which the liquid is introduced into the gas stream. An example of the combination of a spray chamber with a venturi is shown in U.S. Pat. No. 2,579,282.
In each of the above types of air pollution control devices, it is necessary to provide fans or blowers to drive the contaminated gas through the system.
In the venturi jet scrubber, the motive force is provided by a cold water ejector mounted generally on the axis of the converging section of the venturi and no additional fans or blowers are required. Water is pumped through the ejector nozzle where it is broken up or atomized into droplets which are then mixed with the gas. Driving is accomplished by an exchange of momentum between the driving water and the driven gas and, simultaneously, the particulate in the gas is removed by collision or impaction with the water droplets. A venturi jet scrubber of the type here described has long been available from the Koertrol Corporation and is designated as the "Type 7010" scrubber. Additional examples of venturi jet scrubbers are shown in British Pat. Nos. 1,227,499 and 881,437, German Pat. No. 280,088 and U.S. Pat. No. 3,385,030. However, as with the other scrubbing devices referred to above, the effectiveness of the venturi jet scrubber drops as the particle size falls below 2 microns and particularly as the size falls below 1 micron. Thus, where it is necessary to maintain a high cleaning efficiency with respect to fine particulate, even the venturi jet scrubber is inadequate.
As pointed out above, in order to increase the cleaning efficiency with respect to fine particulate, it is necessary to increase the flow of water, decrease the droplet size, increase the number of available droplets, increase the relative velocity between the droplets and the gas or some combination of these factors. Each of the above alternatives, which requires an increase in the input energy, has been applied, in one form or another, to produce a high-energy wet scrubber.
The use of hot water under high pressure to form a hot water ejector was first applied as a drive for wind tunnels (see U.S. Pat. Nos. 2,914,941 and 3,049,005). Later, the hotwater drive was applied to test stands for jet engine testing (see O. Frenzl "Hot-Water Ejector for Engine Test Facilities," Journal of Spacecraft, May-June 1964, Vol. 1, No. 3, pp. 333-338). In both of these applications particulate contained in the air or in the combustion products from the jet engines was removed along with the water from the system.
More recent developments in the art of pollution control equipment have demonstrated the feasibility of combining into a system various of the particulate control devices described above. See, for example, U.S. Pat. No. 3,894,851. Thus, it has been common to use a spray chamber followed by a cyclone separator or a venturi scrubber; a venturi jet scrubber followed by a separator; or two venturi jet scrubbers followed by a separator. A scrubber system of the latter type is described in an article by L. S. Harris entitled "Fume Scrubbing With the Ejector Venturi System" (Chemical Engineering Process, Vol. 62, No. 4, pp. 55-59, April 1966). A system employing a quenching chamber, a venturi scrubber and a spray cooling tower is described in an article by Willet and Pike entitled "The Venturi Scrubber for Cleaning Oxygen Steel Process Gases" (Iron and Steel Engineer, July 1961, pp. 126-131). As shown in that publication, a pressure drop in the venturi of 50 to 60 inches of water was required to attain cleaning levels of 0.02 to 0.03 grains per standard cubic foot of dry gas. The combination of a hot water drive with a cyclone separator as an air pollution control system where hot water in its liquid state is both the source of the driving energy and the cleaning medium is also shown in U.S. Pat. Nos. 3,613,333, 3,704,570 and 3,782,074.
U.S. Pat. No. 3,852,408 issued to applicants' assignee discloses a process for removing particulate and gaseous sulfur dioxide in an apparatus comprising a spray chamber for conditioning the contaminated gas and removing large particulate, a hot-water drive and chemical injection unit for driving the gas and capturing the remaining particulate and the sulfur dioxide reaction products in water droplets, and a cyclone separator for separating the water droplets and sulfur dioxide reaction products from the cleaned gas.
Although the process described in U.S. Pat. No. 3,852,408 is effective to remove both particulate matter and sulfur dioxide in a single system to levels not heretofore possible, difficulties were experienced in handling, on a continuous basis, hot water at high pressures. Furthermore, the cost of heating and treating the required water (i.e. 0.3 to 0.5 lb. water per lb. of contaminated gas) is substantial even though these costs are lower than those of alternative systems. Further development led to the concept of separating the driving and cleaning functions. The driving function was performed by a steam or air ejector, while the cleaning function was performed by atomized water (which could be unheated and untreated). This improved process is disclosed in U.S. Pat. No. 3,852,409, also issued to applicants' assignee. The process of U.S. Pat. No. 3,852,409 simplified the process of removing either particulate or sulfur dioxide or both from a contaminated gas stream by eliminating the use of hot water which was difficult to handle. The new process also was more efficient since, among other things, there were substantially reduced requirements for water heating and treating.
Since the development in the early 1970's of the processes disclosed in U.S. Pat. Nos. 3,852,408 and 3,852,409, the cost of energy, particularly in the form of steam, has increased greatly while the ambient air standards have tended to become even more restrictive. It is, therefore, a principal object of the present invention to provide an apparatus for removing pollutants from a contaminated gas stream or the ambient air with a minimum consumption of energy. Another object of the invention is to provide a system in which either particulates or particulates and gaseous pollutants may be removed from a contaminated gas stream. A further object of the invention is to provide a system capable of handling efficiently wide variations in the flow of a contaminated gas stream by the use, in some circumstances, of a modular construction. A still further object of the invention is to provide a simplified system for the removal of particulates and gaseous pollutants from a contaminated gas stream.